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This study characterized changes in levels of mRNA and protein for endometrial oestrogen receptors (ERs) and progesterone receptors (PRs) during luteolysis and maternal recognition of pregnancy. For cyclic and pregnant ewes, endometrium was collected on days 10, 12, 14, or 16 post-oestrus (4 ewes/day for each status) for the measurement of ER and PR mRNA and protein. The amount of receptor mRNA is expressed in relative units above background, measured from radiographs of dot-blot hybridization of total endometrial RNA with ER and PR cDNAs. At hysterectomy, jugular vein blood samples were collected and assayed for progesterone, total corpus luteum weight was recorded and, in vitro, endometrial oxytocin-stimulated inositol phosphate formation was estimated. In pregnant ewes, plasma progesterone increased gradually between days 10 and 16 (P<0·01), corpus luteum weight was stable at approximately 08 g and oxytocin did not stimulate endometrial formation of inositol phosphates in vitro. In contrast, in cyclic ewes, plasma progesterone decreased from day 10 to day 16 (P<0·01), corpus luteum weight decreased after day 14 to approximately 0·48 g (P=0·05) and oxytocin stimulated an increase of approximately 1300% in the endometrial formation of inositol phosphates on day 16. cDNAs specifically hybridized with 1·6 and 31 kb transcripts for PR mRNA and a 6·5 kb transcript for ER mRNA. In cyclic ewes, the amount of PR mRNA increased from day 10 to maximum levels on days 14–16. The number of PRs decreased from day 10 (225 pmol/mg DNA) to day 12 (0·98 pmol/mg DNA) and then increased from day 14 to day 16 (2·8 pmol/mg DNA). In pregnant ewes, PR mRNA levels were greatest on days 10–12 and decreased by approximately 50% by day 16. In contrast, the number of PRs was relatively unchanged from day 10 to day 16 (1·53 to 103 pmol/mg DNA). In cyclic ewes, the amount of ER mRNA was lowest at day 10 and increased fivefold by day 16. The number of ERs remained relatively unchanged from day 10 to day 14 (607 pmol/mg DNA) and increased by day 16 (1612 pmol/mg DNA). In pregnant ewes, ER mRNA decreased by approximately 80% from day 12 to day 16. Similarly, the number of ERs decreased from day 10 to day 16 (5·41 to 205 pmol/mg DNA). Correlations between ER mRNA and PR mRNA (r=0·68), ERs and PRs (r = 0·50) and ER mRNA and ERs (r=0·50) were high (P<0·01). PR mRNA and PRs, PR mRNA and ERs, and ER mRNA and PRs were not correlated (P>0·1). Pregnancy had the apparent effect of stabilizing the number of endometrial PRs and inhibiting ER production by decreasing both the amount of ER mRNA and ER protein.

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